1. Field of the Invention
The present invention relates to a magnetic recording medium, a method for producing the same, and a magnetic recording apparatus. In particular, the present invention relates to a magnetic recording medium which is excellent in thermal stability, which makes it possible to perform the high density recording thereon, and which makes it possible to perform the reproduction at a low noise level. The present invention also relates to a method for producing such a magnetic recording medium and a magnetic recording apparatus.
2. Description of the Related Art
In recent years, the multimedia comes into widespread use in accordance with the development of the advanced information society. It becomes possible to process not only the text information but also the voice and image information at high speeds. One of the multimedia is represented by the magnetic recording apparatus. The magnetic recording apparatus includes a plurality of magnetic recording media which are rotatably installed onto a spindle. The development is advanced for the magnetic recording medium to be used for the magnetic recording apparatus in order to miniaturize the apparatus while further improving the recording density.
The magnetic recording medium comprises a substrate and a magnetic film formed thereon. Information is recorded on the magnetic recording medium by forming magnetic domains having specified directions of magnetization in the magnetic film. The conventional magnetic recording medium has used the in-plane magnetic recording method in which the magnetization is directed in the in-plane direction of the film. In this method, the high density recording on the magnetic recording medium was successfully achieved by making the magnetic crystal grains fine and minute and reducing the magnetic interaction between the respective grains. However, a problem has arisen such that the thermal stability of the recording magnetization is lowered due to the fine and minute magnetic crystal grains and the reduction of the interaction between the respective crystal grains.
The perpendicular magnetic recording method, in which the recording magnetization is directed in the vertical direction to the substrate, has been suggested as a method for solving the problem as described above. When this method is used, then the magnetostatically stable state is given between the adjoining recording bits, the thermal stability is improved, and the recording transition area is sharp. Further, when a layer formed of a soft magnetic material (hereinafter referred to as “soft magnetic backing layer”) is added between the substrate and the recording layer of the magnetic recording medium based on the perpendicular magnetic recording method, it is possible to obtain a steep magnetic field to be applied to the recording layer. Therefore, it is possible to use a material having higher magnetic anisotropy for the recording layer. Accordingly, the thermal stability is further enhanced, and it is possible to perform the high density recording.
The studies have been principally directed to the CoCr-based alloy as a material for the recording layer of the magnetic recording medium based on the perpendicular magnetic recording method. The recording layer, which is formed of the CoCr-based alloy, has a two-phase separation structure composed of the crystal grains with a high Co concentration having the ferromagnetism and the non-magnetic crystal grain boundary with a high Cr concentration. The magnetic interaction between the crystal grains can be blocked by the non-magnetic crystal grain boundary. Therefore, the low noise level of the medium required for the high density recording has been realized.
In order to obtain a magnetic recording medium on which the high density recording is successfully performed at a lower noise level as compared with the magnetic recording medium based on the perpendicular magnetic recording method which uses the CoCr-based alloy for the recording layer, it is necessary to further enhance the thermal stability of the magnetization. For this purpose, it is necessary to use a material having higher magnetic anisotropy as compared with the CoCr-based alloy for the recording layer. Such a material may be exemplified, for example, by a multilayer film (artificial lattice film) obtained by alternately stacking Co and Pd or Co and Pt, and an ordered alloy composed of Fe and Pt or Co and Pt. However, the materials as described above have strong magnetic interaction between the crystal grains. Therefore, the following problem arises. That is, the size of the minimum magnetic domain is increased, and the transition noise is increased in the recording transition area between the adjoining recording bits during the recording.
In order to solve the problem as described above, a magnetic recording medium has been disclosed, for example, in Japanese Patent Application Laid-open No. 2002-25032 (pp. 2 to 4, FIG. 2), corresponding to U.S. Patent Application Publication No. U.S. 2002/0015864 A1, in which B element and O element are contained in a recording layer formed of an artificial lattice film. In this magnetic recording medium, 1 to 15 at. % of B is contained in the artificial lattice film. Accordingly, the crystalline isolation is improved for the crystal grains in the recording layer, and thus the transition noise is reduced. In this magnetic recording medium, the isolation of the crystal grains in the recording layer is further improved by containing 1 to 30 at. % of B in an underlayer of the recording layer.
As disclosed in Japanese Patent Application Laid-open No. 2002-25032, the following problem has arisen in the magnetic recording medium in which B is contained in the recording layer. That is, when B is contained in an excessive amount (for example, B having a value higher than 15 at. % in Japanese Patent Application Laid-open No. 2002-25032) in the recording layer in order to further enhance the isolation of the crystal grains in the recording layer, then B enters the crystal grains, the perpendicular magnetic anisotropy is deteriorated, and the transition noise is increased.